Characterization, sources, and risk assessment of PAHs in borehole water from the vicinity of an unlined dumpsite in Awka, Nigeria

Polycyclic aromatic hydrocarbons (PAHs) are contaminants of interest in the ecosystem due to associated health risks. Therefore, their detection in the environment is important. In this regard, the risk assessment of PAHs in borehole water near the unlined dumpsite in Anambra State was investigated. Samples of borehole water (16 each) were collected from the study and control areas during both seasons. The PAH concentrations in the borehole water samples were analyzed using gas chromatography. The mean PAH concentration in the study and control samples for the wet season varied from BL–7.65 µg/L to BL–2.98 µg/L, respectively. The study samples' dry season values ranged from BL to 3.33 µg/L, while control samples ranged from BL to 1.87 µg/L. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sum$$\end{document}∑PAHs for the wet and dry seasons varied from 5.8 to 13.94 µg/L and 4.25 to 10.09 µg/L for study and control samples, respectively. The four and five rings PAH were the most dominant group in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sum$$\end{document}∑ PAHs for the study and control samples, respectively. Diagnostic ratios suggested pyrolytic and petrogenic sources for both locations. The cluster analysis showed different sources of the congeners in the samples. The non-carcinogenic risk showed no possibility of risks via dermal and ingestion routes. In addition, the possibility of cancer risks via ingestion routes was doubtful. The carcinogenic risk index through dermal contact exceeded the acceptable limit for adults and is at a tolerable limit for children, indicating potential threats to humans, with adults more susceptible to cancer risks. Therefore, this study recommends that sanitary dumpsites be constructed for waste disposal and implementation of environmental laws to prevent underground water pollution and the environment.

The study samples were collected within 152-213 m from the dumpsite area, while a distance of 619-788 m away from the study samples was used to collect the control samples. Before sampling, glass sample bottles were washed with detergent, rinsed with distilled water, and dried in an oven. Properly cleaned glass bottles were used to collect borehole water samples. The 16 samples from each location were combined to form a homogenous sample representing the samples collected from a particular location. The homogenized water samples were stored in the refrigerator at 4 • C before analysis.
Chemicals used for the analysis. A standard mixture of 16 US EPA priority PAHs was procured from Accustandards Inc (USA). Analytical grade dichloromethane, acetonitrile, acetone, n-hexane, and anhydrous sodium sulphate were acquired from Sigma-Aldric, USA.

Preparation of borehole samples and clean-up.
The analysis was carried out using the method 45 .
10 mL of the sample was extracted with 200 mL of dichloromethane. The separation of the mixture was carried out using a separating funnel and was concentrated with the aid of a rotary evaporator. The concentrated sample was analyzed by adding 1 mL of acetonitrile. Residue cleaning was performed using an 8 mL (12 mm 5 cm long) glass chromatography column from Restek, USA. The sample was passed through a chromatographic column loaded with 14 g of activated silica gel (60-100 mesh) deposited with glass wool and anhydrous Na 2 SO 4 (2 g). It was conditioned with 7 mL of n-hexane. The concentrated extract was dissolved in 2 mL n-hexane and loaded into the column. The eluate collected was concentrated using a rotary evaporator. The concentrated eluate was used for analysis after dissolving it with 1 mL of acetone.
Quantitative analysis of PAHs. A Buck Scientific M910 gas chromatograph (USA) coupled with a flame ionization detector was utilized for the analyses. A column type HP 88 with dimension (100 m × 0.25 μm thickness) CA., USA, and an on-column automatic injector were used for PAH detection. Helium (carrier gas) with a maintained flow rate of 1.5 mL/minand oven ramprates of 6 °C/min was utilised for the experiment. The oven temperature was programmed to start at 70 °C and increase to 300 °C. The detector was operated at 325 °C. The injector temperature was set at 280 °C. The inlet temperature was set at 290 °C. 1 μL was the injected volume using a split mode with a ratio of 5:1 46 .
Validation of experiment. 100 mL of borehole water sample collected from a different location (blank) was spiked with 1 mL standard PAH solution. It was properly extracted using 200 mL of dichloromethane, clean up of the GC column was done, and the concentrated extract analysed for PAHs using the procedure stated by Omores et al. 46 . The intraday and interday precision was determined by analyzing the prepared samples on the same day and three different days, respectively. Triplicate analyses were done for the recovery experiment. The overall average recovery rates were 90.6-98.8% (Table 2) and within acceptable limits 18,47 . The limits of detection for the PAHs are also shown in Table 2. The analyte peak was identified by comparison of sample retention time values with those of the standard compounds 48,49 . All calibration curves of the tested PAHs were found to Health risk assessment. The study calculated health risks using the benzo (a) pyrene toxicity equivalent ( BaP eq ). The BaP eq was computed using the expression in (Eq. 1) 18,50 .
C e and TEF e indicates PAH's concentration and toxicity factors, respectively ( Table 3). The health risk was also calculated using the benzo (a) pyrene mutagenic equivalent quotients ( BaP Meq ). The BaP Meq was computed using (Eq. 2) 51,52 .
C e and ME F e indicates the concentration and mutagenic factors of corresponding PAH (Table 3). Health risks were calculated using risk equations for dermal and ingestion pathways 53,54 .
The average daily dosage by dermal interaction ( ADD dermal ) was evaluated for non-carcinogenic risks using (Eq. 3).
(1) where ADD dermal corresponds to the average daily dosage by dermal interaction (mg/kg/day); C represents levels of PAHs (mg/L); EF refers to the frequency of exposure (350 days/year); ED refers to the duration of exposure (20 years and 6 years for adult and child respectively) 55 ; BW denotes for the body weight (80 kg and 15 kg corresponds to the adult and child weight respectively) 55,56 ; AT denotes average life span (7300 days and 2190 days for adult and child respectively) 57 ; SA represents the dermal surface area (19,652 cm 2 and child: 6365 cm 2 for adult and child respectively) 55 ; ET denotes the exposure time of shower and bathing (adult: 0.71 h/day; child: 0.54 h/day) 55 ; CDI ingestion is the chronic daily intake (mg/kg/day); IR stands for the water ingestion rate (adult: 2.5 L/day; child: 0.78 L/day) 55 . The Kp (cm/hr) stands for permeability coefficient (Nap: 0.047; Phen: 0.14; Fla: 0.22; BaA: 0.47; BbF: 0.7; BaP: 0.7; DbahA: 1.50; Pyr: 0.324) 58 ; CF represents conversion factor (L/1000 cm) 58,59 .
The HQ and HI, which represent hazard quotient and hazard index, were calculated for individual PAHs using the following equations 60,61 .
CSF stands for the cancer slope factor, which was extrapolated by multiplying the CSF for BaP (7.3 mg/kg/ day) by the toxic factor of individual PAHs 53 .
Statistical estimation. Microsoft Office was used for calculating the mean standard deviations of the sample results. A hierarchical cluster dendrogram was used to assess the relationship between the PAH parameters using OriginPro 9.0. Pearson's correlation analyses at 0.05 significant levels assessed the results between the study areas of the boreholes using SPSS software.
Ethical approval. All the authors have read, understood, and complied as applicable with the "Ethical responsibilities of Authors" as found in the Instructions for Authors.

Results and discussion
Levels of PAHs in the sample. The mean results for the borehole water samples are illustrated using Table 3. The data in Table 4 shows the comparative study results of the study area with other regions. The borehole samples recorded different PAH concentrations for both locations, confirming the pollutants' ubiquitous nature. Some values were below limit (BL) in the experiment. The wet season PAH mean values varied from BL to 7.65 µg/L for study locations and BL to 2.98 µg/L for control locations. The level of PAHs in the dry season varied from BL to 3.33 µg/L for study locations, while control areas varied from BL to 1.87 µg/L. The wet season values were higher than the dry season, which might be attributed to the leaching of pollutants from the refuse dump and surface runoff through rainfall [62][63][64] . The mean study sample values (Fig. 2) were higher than the control sample values due to the infiltration of leachates from the dumpsite 12 . The BaP values were lower than the permissible limits of 200 μg/L and 700 μg/L for both locations 59,65 . The values of BaP, which ranged from 1.2 to 4.3 µg/L, were higher than the study sample's values 62 . The values obtained in Tehran, Iran, which ranged from BL to 0.01 μg/L, were lower than the present study 66,67 .
The low molecular weight PAHs occur mainly in lower concentration values as a result of their high volatility and dissolution 62 . Naphthalene which is a low molecular weight PAH is mainly from petrogenic sources  77 .
Higher molecular weight PAHs comprising four or more aromatic rings were also detected in the borehole water samples. Pyrene had the highest concentration (7.65 μg/L) of individual PAHs in the borehole water. BbF is a colourless, aromatic hydrocarbon with five fused rings formed through incomplete combustion of organic matter 51 . The individual PAHs mean values of BbF which ranged from 3.1 to 3.13 μg/L for the study sample and 1.87 to 2.98 μg/L for control samples, were lower than the values reported in a study conducted by Onydinma et al. 62 .
DbahA is a five-fused benzene ring produced from the incomplete combustion of organic matter 43 . Worthy of note is that DbahA was the least detected PAH and occurred at a relatively lower concentration in the samples with a range of BL-0.13 μg/L. The values of Fla ranged from 0.05 to 0.6 μg/L and 0.08 to 0.37 μg/L for study and control samples, respectively. The obtained values were lower than the values reported by Edet et al. 69 . Table 4 compares the total PAH levels in borehole water samples located within the dumpsite with borehole samples located in other dumpsites in other regions. High levels of PAHs higher than the study areas were found in Abia and Imo, Nigeria 62   www.nature.com/scientificreports/ The PAHs found in the borehole water samples comprised low and high molecular weight PAHs, which was reported previously 7,12,18 . The PAH values revealed that the borehole samples were contaminated with varying PAH concentrations due to their proximity to landfill leachates for both locations, which agrees with previous literature 6,7,12 . The summation level of PAH values ( PAHs) obtained in Fig. 3 showed that the study samples had the highest values of 13.94 µg/L and 10.09 µg/L for wet and dry seasons, respectively. The study samples ( PAHs) values were greater than the control samples for both seasons, which was attributed to the runoff of leachates from the dumpsite due to its proximity to the study samples 12 .
The wet season HMW and ( LMW) values were greater than the dry season values due to more contamination of the borehole water through leachate runoff 12,80 . The carcinogenic PAHs (cPAHs) levels were evaluated from the borehole water results in Table 3 and Fig. 4. The cPAHs in the wet season showed 28.19% for study samples and 58.28% for control samples, while the study and control samples showed 41.63% and 51.11%, correspondingly, during the dry season. The cPAHs revealed that the study sample values were greater than the control samples in both seasons. It was attributed to the discharge of leachate from the refuse dump that contributed to the pollution of the borehole samples 12,18 . In the study areas, the levels of PAHs during the wet season followed: Pyr > BbF > Nap > BaA > Phen > Acp > BghiP > Flur, BaP > Acy, Fla, BkF > DBahA, while the control location was BbF > Acp > Nap > Phen > BaA > Flur > Fla > Acy > Bap > BghiP > BkF, Pyr, DBahA. The dry season levels of PAHs obeyed this order for the study site: Pyr > BbF > Nap > BkF > Fla > BaA > BaP > Acy, BghiP > DBahA > Acp, Flur, Phen, while the control site obeyed this order: BbF > Acy > Phen > Fla > BaA > Pyr, Nap > BaP, BghiP > Acp, BkF, Flur, DBahA.
In the wet season (Table 5), the correlation between the study and control locations showed a weak positive correlation (r = 0.229, p = 0.432). The p value (p > 0.05) was non-significant, which implied that the difference between the study and control samples was not statistically significant. In the dry season, the concentration of PAH values has a moderate positive correlation between the study and control sample values (r = 0.535, p = 0.048), which revealed a significant difference between both samples. The correlation values between the study samples (r = 0.880, p = 0.000) and control samples (r = 0.929, p = 0.000) for both seasons showed a significant positive www.nature.com/scientificreports/ correlation. It indicated that the study and control sample's PAH levels in the wet season were higher in the dry season due to the influx of leachates from the dumpsite. Consequently, the over-dependence on these boreholes by individuals residing around the dumpsite for a long time may result in several human health conditions. Characterisation of ring size. Figures 5 and 6 show the ring size arrangement in the borehole water samples for both locations. The 4-ring PAHs recorded the maximum value (59.92%) during the wet season at the study location, while 52.3% was observed at the 5-ring PAHs. The maximum value of 41.6% was observed during the dry season for 4-ring PAHs, while the 5-ring PAHs recorded a 46.4% maximum value. The 6-ring PAHs recorded the lowest value of 0.52% in the wet season. The ring size profile generally showed that the HMW-PAHs had a higher percentage contribution than the LMW-PAHs. This finding isin agreement with similar studies 12,69,81 but not in agreement with the study conducted by Aderonke et al. 82 and Adedosu et al. 81 where the LMW-PAHs were the dominant PAHs. The dominant high molecular weight PAHs were attributed to the incomplete combustion of organic materials and solid wastes from the dumpsite 81 . The LMW-PAH's presence in the ring structures is linked to the emission of oil spills and non-combustible matter 81 . Table 5. Pearson correlation between study area parameters across both seasons. **Correlation is significant at the 0.01 level (2-tailed). *Correlation is significant at the 0.05 level (2-tailed).  www.nature.com/scientificreports/ Source identifications of PAHs. Isomeric ratios of PAH have been applied in the determination of possible input sources of PAH and their transport properties 33,83 . In the present study, PAH source identification was carried out using diagnostic ratios 50 . Fla/Pyr PAHs ratio < 1 implies petrogenic, while > 1 implies pyrolytic 84 . BaA/228 ratio showing < 0.2 suggests petrogenic, while 0.2-0.35 implies pyrolytic sources 85 . The ratio of ΣLMW/ΣHMW was < 1 for the study and control locations (Table 6), suggesting a dominant pyrolytic source due to incomplete combustion of refuse or biomass 18,85,86 . Also, BaA/228 ratio showed a petrogenic input for both the study sample and control sample locations. The Fla/Pyr diagnostic ratios suggested that study sample locations were from petrogenic sources, while control sample locations confirmed pyrolytic sources. Generally, the PAH contamination in both study areas originated from pyrolytic sources, largely due to incomplete combustion of biomass, discharge of untreated leachates and surface runoff, while the petrogenic sources were due to combustion of petroleum products and oil spills. A predominant petrogenic source was observed in the study sample, while in the control sample, the pyrolytic sources were the dominant PAHs source.
PAHs cluster analysis. The hierarchical cluster dendrogram (HCD) showed that the PAH congeners in the borehole samples during the wet season were grouped into four clusters (Fig. 7). Acy, BkF, NaP, and BbF are in the first cluster, Fla and Flur in the second cluster, BaA,Pyr, Ant, DBahA, in the third cluster, while BaP, BghiP, and Phen are in the fourth cluster. The first cluster mainly comprises 5, 3, and 2-membered ring PAHs, while the second comprises 5 and 3-membered PAHs. Fluorene, fluoranthene, chrysene, and pyrene are markers for oil combustion 87 . The third and fourth clusters comprised 5, 4, and 3-membered rings and 5, 6, and 3-membered rings, respectively.
During the dry season, the PAHs were grouped into two main clusters Acy, BaA, Pyr, and BaP are in one cluster, while the rest are in the second cluster (Fig. 8). The first cluster mainly comprises 5, 3, and 4-membered ring PAHs. The second cluster comprises 6,5,4, 3, and 2-membered ring PAHs. The difference in the clustering during the wet and dry seasons could be attributed to the leachate runoff caused by rainfall during the wet season (seasonal variation) and concentrations of the PAHs congeners where most were undetectable during the dry season 6,88 . Toxicity and mutagenic equivalent assessment. The summation of the benzo(a) pyrene toxicity and mutagenic equivalent (TEQs and MEQs) are presented in Fig. 9. The TEQ value for the sample study in the wet season was 0.49, while for the control study was 0.39. The TEQ value for the dry season was 0.57 and 0.32 for the study and control samples, respectively. The MEQ values in the wet season were 0.97 for the study sample and 0.83 for the control sample. The dry season values were 1.03 and 0.61 for the study and control samples, respectively.
The TEQ and MEQ values for the study sample locations were higher than those at the control locations in both seasons, which might be attributed to the infiltration of pollutants from the refuse dump 6,7,12 . The BbF followed by BaA contributed significantly to the TEQ values. The BbF followed byBaP contributed significantly PAHs risk assessment of borehole water samples. Hazard quotient (HQs) values obtained from the average daily dose ( ADD derm ) are shown in Table 7. The HQ and HI values obtained via skin absorption were < 1 for age categories and locations. Therefore, the possibilities of non-carcinogenic risks are very unlikely 21 . The HI values for the child were higher than the adult, which agrees with previous work 62,69,86,90 . Cancer risk through dermal exposure is shown in Table 8. ILCR values (1.E−06) are deemed satisfactory, above 1.00E−05 but lesser than 1E−04 are tolerable, while values ≥ 1.0 ×10 −4 indicate severe threats 91 . The ILCR values were less than 1 ×10 −4 . The hazard indices showed that the adult HI was predominantly higher than the child HI for both locations, confirmed in a similar report 75 . HI values for the adult age category were above the threshold limit, while HI values for children were within the tolerable limit. The sample study HI values were higher than the control sample values. The overall assessment showed that the borehole water samples are unfit for washing, bathing/showering. Adults are more prone to exposure to cancer health risks than children, which was in agreement with previous work 75 .
HQ and HI values for non-carcinogenic PAH exposure through the ingestion route are illustrated using Table 9. The hazard quotient values were < 1, which shows no chance of a non-carcinogenic effect 88,92,93 . The HI values for the study location were higher than the HI of the control location. The HI values were less than 1, which suggested no chance of contacting non-carcinogenic health risks.  www.nature.com/scientificreports/ The ILCR and HI values via the ingestion route are obtained in Table 10. The ILCR values were < 1E−04. The HI values were within the tolerable limit 1E−05. The adult HI was higher than the child HI, which showed that the adult has more chances of exposure to cancer risk through bioaccumulation 75,86,90 .

Conclusions
The borehole water samples were contaminated with PAHs through leachate runoff from rainfall. The total PAH concentration values obtained showed that the study sample was predominantly greater than the control sample due to its closeness to the dumpsite. The PAH levels in the borehole water samples were greater in the wet than in the dry season due to leachate infiltration from the dumpsite. The predominant ring in the study location was the 4-ringed PAHs, whereas the most dominant PAH group was the 5-ringed PAHs. The least dominant PAH group was the 6-ringed PAHs for both seasons and locations. The diagnostic ratios suggested both locations had mixed sources (petrogenic and pyrolytic). The TEQ and MEQ values were greater in the study samples than in the control samples. The individual PAH contributions to the TEQ and MEQ could trigger carcinogenic and non-carcinogenic health effects. Non-cancer risks seem unlikely for dermal contact and ingestion exposure routes. Carcinogenic risk through dermal contact exceeded the threshold limit for an adult and was lower for a child at the tolerable limit. Adults would be more susceptible to cancer risk than children. The HI values for carcinogenic risks through the ingestion pathway were within (1.0 ×10 −5 ) the acceptable limits for the adults and the children categories in all the locations. Based on the study's findings, there is a dire need to protect the environment and make it suitable for human lives by controlling the indiscriminate release of pollutants which often bioaccumulate to toxic levels if unmonitored. In addition, we recommend that the borehole water be treated before use to avoid health-related risks through domestic usage.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.   www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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